Make graphql a peer dependency

[ntharim]

I also updated graphql to 0.6.0 since there are no breaking changes and all tests passed with it. However if you don't want to update yet I'll change it back to 0.5.0.

Fixes #50.

[apollo-cla]

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[ntharim]

Reverted to graphql@0.5.0 because of npm install failing on Node v4.

[helfer]

Thanks! I don't think the npm install issue was related, but we can update to 0.6.0 in a separate PR. (Travis just fails sometimes, and you have to restart the job)